Plastics material mesh structures

ABSTRACT

In order to provide wider biaxially-stretched geogrids having high primary direction (PD) strength, crotch-forming zones of a starting material which is at least 2.0 mm thick have protuberances ( 6 ). On strectching, PD orientation passes right through the junction ( 27 ) but the protuberance ( 6 ) causes the orientation ratio of the crotch edge to decrease significiantly as orientation enters the central part of the crotch edge. After a secondary direction stretch of at least about 1.5:1, the thickness of the central part of the crotch edge is not reduced by more than about 20%. The stretching does not reduce the thickness of any point along notional ridge lines ( 31 ) to such an extent that the ratio of finished thickness to starting thickness at that point mid-point. In the biax geogrid ( 25 ), the junction mid-point is significantly thicker than the PD is less than about 80% of the ratio of finished thickness to starting thickness of the junction strand mid-point. The thickest part of the crotch is greater than about 80% of the thickness of the mid-point of the junction ( 27 ). The junction ( 27 ) has a generally square or rectangular central zone with narrow projections ( 28 ) at the corners extending outwards to the crotch edges.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing abiaxially-stretched plastics material mesh structure, suitable forgeotechnical and other civil engineering uses; a mesh structure employedfor such uses can be referred to as a geogrid. The mesh structure isunbalanced, ie has a greater strength in a primary direction (PD) thanin a secondary direction (SD) substantially at right angles to the PD.The method comprises providing a starting material which has a thickness(defined below) of at least about 2 mm and has a pattern of holes on anotional substantially square or rectangular grid whose axes aresubstantially parallel to the PD and to the SD respectively, the sidesof PD end portions of at least some of said holes being defined bycrotch-forming zones. The starting material is of square construction;ie the PD and the SD are aligned with the two orthogonal axes of theeventual product. PD stretch is applied to form oriented PD strands andto apply some orientation to the notional junction zones (defined below)so that orientation extends into and through the notional junctionzones, from the end of one oriented PD strand to the adjacent end of thealigned oriented PD strand. SD stretch is applied to form oriented SDstrands. At least part of the length of the edges of crotches connectingadjacent sides of adjacent PD and SD strands is oriented in thedirection running round the crotch. The mid-point of the notionaljunction zone (defined below) in the mesh structure is significantlythicker than the mid-point of any oriented strand entering the notionaljunction zone.

The present invention also relates to a biaxially-molecularly-orientedintegral plastics material mesh structure made from a starting materialwhich has a thickness of at least about 2 mm and comprising oriented PDstrands and oriented SD strands, interconnected by junctions withrespective sides of adjacent oriented PD and oriented SD strandsinterconnected by crotches at least part of whose length is orientedwith orientation running in the direction around the crotch. Theorientation of the PD strands extends into and through the junctions.The junction mid-point thickness is significantly greater than themid-point thickness of any oriented strand entering the junction.

The mesh structures of the invention were designed for applications inwhich the main tensile force will be applied in the PD, and the meshstructures have significantly greater strength in the PD than in the SD;for instance, when the mesh structure is attached to a vertical wallfacing in a retained wall geoengineering construction, the main tensileforce will be at right angles to the facing and can be aligned with thePD of the mesh structure.

U.S. Pat. Nos. 4,374,798 and 5,053,264 disclose mesh structures of thegeneral type with which the present invention is concerned. U.S. Pat.Nos. 5,267,816 and 5,269,631 disclose mesh structures which have greaterstrength in the PD than in the SD.

From the point of view of PD strength and failure mode, the beststructure is found to be a uniaxially-oriented (uniax) structure, as forexample strengths of up to at least 160 kN/m width can be achieved witha 5.5:1 PD stretch ratio. However, in order to reduce the time taken tobuild a geotechnical construction whilst maintaining the strength of theconstruction, it is desirable to increase the width of the geogridwithout reducing the PD strength per strand or the economy rating in thePD more than about 5% or 10%. As there is a limitation on the width ofstarting materials, this can only be achieved in manufacture bystretching in the transverse direction (TD). Thus although theimprovements of U.S. Pat. Nos. 5,267,816 and 5,269,631 are primarilydirected towards significantly increasing the machine direction (MD)strength by enabling one to apply greater PD stretches, the presentinvention is more concerned with being able to apply SD stretcheswithout greatly reducing the PD strength per strand or the economyrating, whilst providing a satisfactory creep strain behaviour withadequate long term strength. Nevertheless, it is very difficult toproduce biax products whose PD strength per strand or economy rating issimilar to the PD strength per strand or economy rating of theintermediate uniax material if reasonable stretches are applied in theSD. This is because the SD stretch affects the PD strength of thejunction. Also the SD strands must be sufficiently robust eg toreinforce soil.

It is desirable to have a low SD punch-out in order to leave as muchmaterial as possible in the PD strand-forming zones and produce a highstrength product using not too thick a starting material. However, ifthe SD punch-out is decreased, the PD orientation through the junctionbecomes more difficult to control.

It is desirable that any alteration in the manufacturing techniqueshould produce a mesh structure which appears robust to the eye (as thisis important for sales), and that the mesh structure should have anacceptable failure mode. As a general principle, it is desirable to havefailure in the PD strand and not in the junction as this enables betterprediction of the long term performance of the product.

THE INVENTION

According to the method of the invention, crotch-forming zones in thestarting material have protuberances. Each protuberance may be in theplan view shape of the hole, namely a portion of the crotch-forming zonewhich projects into the respective hole beyond a base line which istangent to each end of the projecting portion. Alternatively or inaddition, the protuberance may be out of the plane of the startingmaterial, being a thickening when approached from the edge of the PDstrand-forming zone and/or when approached from the edge of the SDstrand-forming zone. In the special case where there is no thickeningwhen approaching from the edge of the PD strand-forming zone (definedbelow) and also no protuberance as seen in the plan view shape of thehole, there is a tendency for the orientation to proceed too far aroundthe crotch in the PD stretch. This tendency is avoided by ceasing PDstetching before the orienation has reached the thickening (whichconsidered as one progresses from the PD strand-forming zone will be adecrease in thickness).

The size of said protuberance is such that during the stretch, at leastpart of the edge of the crotch interconnecting adjacent sides ofadjacent oriented PD and oriented SD strands is oriented in thedirection running around the crotch, but the orientation ratio decreasessignificantly as one passes around the crotch edge, either from theoriented PD strand or from the oriented SD strand. In the biax product,the crotch edge either a) has an unoriented part, or b) the thickness ofthe least oriented part of the crotch edge is reduced (or the length ofthe least oriented part of the crotch edge is increased) by no more thanabout 20% (or than about 15%, 10% or 5%, as alternatives) of itsthickness (or length) prior to stretching. The action of stretching doesnot reduce the thickness of any point along notional ridge lines ofmaximum thickness on the product from the junction mid-point to thecrotch edges to such an extent that the ratio of finished thickness tostarting thickness at that point is less than about 80% (or than about85%, 90%, 95%, as alternatives) of the ratio of finished thickness tostarting thickness of the notional junction zone mid-point.

In the biaxially-stretched mesh structure of the invention, thejunctions comprise a central zone which is thicker than thinner zonesadjacent the ends of oriented PD and oriented SD strands. If thejunction interconnects two oriented PD strands entering a junction fromopposite sides and two oriented SD strands entering the junction fromtwo other opposite sides, the junction central zone will be generallysquare or rectangular; the central zone need only be generally square orrectangular and its sides could be concave or convex. The central zonehas a narrow pro jection at the corner extending outwards between saidthinner zones, continuing through the crotch between the oriented PDstrand and the oriented SD strand, and running into the crotch edge.Though the central zone is thicker than said thinner zones, the narrowprojection may be thicker than the central zone. There is no point on anotional line of maximum thickness on the product from the junctionmid-point to the crotch edge having a thickness of less than about 80%(or than about 85%, 90% or 95%, as alternatives) of the thickness of thejunction. mid-point. The central part of the crotch edge zone can beconvex. The mesh material defined above is the product produced from astarting material with flat or nearly flat faces; if a starting materialface or faces diverge markedly from flatness, the product will bealtered correspondingly.

The invention extends to a method of strengthening soil, comprisingembedding in the soil the mesh structure of the invention or the meshstructure produced by the method of the inventio n, and to ageotechnical construction (a composite civil engineering construction)comprising a mass of soil strengthened by embedding therein the meshstructure of the invention or the mesh structure produced by the methodof the invention.

DEFINTIONS

The understanding of several of the following definitions is assisted byreferences in FIG. 1a of the accompanying drawings, but the definitionsare not limited to the shape of hole 1 shown in FIG. 1a.

The term “oriented” means molecularly-oriented. In general, when anoriented strand is referred to, the preferred direction of orientationis longitudinal of the strand.

“Uniax” and “biax” mean uniaxially-oriented and biaxially-oriented,respectively. Substantially uniaxially-oriented means that on thesurface of the structure, there has been extension of the mgterial inone direction but no substantial resultant extension of the material inthe direction at right angles.

In relation to a mesh structure, “biaxially-oriented” means that themesh structure has been stretched in two directions generally at rightangles to each other.

The “orientation ratio” is the stretch ratio in a localised area.

The holes in the starting material may be through-holes or blind holes.If the holes are blind, the film or membrane in the hole will eitherrupture on stretching, or may remain as a thin membrane.

A “tangent line” is a notional line tangent to the ends of the holes oneither side of a strand-forming zone, whether or not the hole is inaccordance with the invention. FIG. 1a shows “SD tangent lines” 2tangent to the PD ends of the holes 1 and “PD tangent lines” 3 tangentto the SD sides of the holes 1.

In extruded starting materials and in embossed or moulded startingmaterials, the holes are not normally vertical sided (ie perpendicularto the plane of the starting material). For extruded starting materialsas a good approximation, the tangent line can be taken as the notionalline tangent to the plan view see-through, ie the minimum hole size asviewed normal to the plane of the material, but ignoring film orfeathered edges. For embossed or moulded starting materials, where theholes normally have sloping sides, as a good approximation, the tangentline can be taken as the notional line tangent to a point half way upthe side of the hole, or, if the slope is different on each face,tangent to a point half-way between points half-way up the respectiveslopes, films or membranes in blind holes being ignored.

The “notional junction zone” 4 is the zone of the starting materialdefined between the respective pairs of SD and PD tangent lines 2, 3. Inthe mesh structures, the notional junction zones are the zones of thesurfaces of the structure which have been formed from the notionaljunction zones of the starting material.

When the plastics starting material is biaxially stretched in twodirections substantially at right angles, the PD and SD, oriented PDstrands and oriented SD strands are formed, extending in directionssubstantially at right angles to each other. The PD and SD strands areconnected at junctions. At the junctions, the edge zone of each PDstrand is connected to the edge zone of the respective SD strand by acontinuous zone of plastics material which is located at the edge of thejunction in the product and is formed from plastics material at thecorner of the junction-forming zone in the starting material. Thiscontinuous zone of plastics material is termed the “crotch” herein. The“direction running around the crotch” as used herein and in the appendedclause is the curved direction extending between adjacent orientedstrands, either from the SD strand to the adjacent TD strand, orvice-versa.

A “crotch-forming zone” 13 is the zone adjacent the corner of therespective notional junction zone 4, lying between a respective hole 1,the adjacent PD tangent line 3 and the adjacent SD tangent line 2.

A “PD strand-forming zone” 11 is the zone between respective holes 1 andat its ends between respective crotch-forming zones 13, which extendsfrom the SD tangent line 2 tangent to the first PD ends of therespective holes 1 to the SD tangent line 2 tangent to the other, secondPD ends of the holes 1.

An “SD strand-forming zone” 9 is the zone between respective holes 1 andbetween respective crotch-forming zones 13, which zone extends from thePD tangent line 3 tangent to the first SD ends of the respective holes 1to the PD tangent line 3 tangent to the other, second ends of the holes1. It is not necessary that orientation will pass along the whole of theSD strand-forming zone, and normally the very ends will not be stretchedout.

A notional line of maximum thickness is a notional line taken from apoint of maximum thickness on one edge of the structure to a point ofmaximum thickness on another edge of the structure and joining togetherpoints of maximum thickness in between.

“Strictly uniplanar” means that the material or structure is symmetricalabout a median plane parallel to its faces. In general, a uniplanarstarting material will give a uniplanar struture when stretched.

“Substantially uniplanar” means that the material or structure does notdeviate so much from strict uniplanarity that orientation is notcomparable on each face of the biax product.

A “strictly flat” starting material has monoplanar, parallel faces.

The “starting material” is the material immediately before initiation ofthe first stretch. However, when considering whether there is athickening, the material should be as it would be without theapplication of the cold grooving technique described in U.S. Pat. No.4,590,029, if such a technique has been applied, as the technique haslittle effect on the final orientation behaviour.

The “pitch” of holes is the distance apart of the centres of the holesin a specified direction.

“Punch-out” is the ratio of the maximum dimension of the holes (iebetween the respective tangent lines) in a specified direction to thepitch of the holes in the same direction, whether or not the holes havebeen formed by punching or by another procedure which may not eveninvolve material removal.

The terms “thick” and “thin” refer to the dimension normal to the planeof the material or mesh structure. Unless otherwise specified, thethickness is tne distance between the extreme faces at the thickestpoint. However, raised edges or tapered or feathered edges are ignored,as well as any minor grooves in the surface and any irrelevantprojections from the surface.

The dimension “t” is the mean thickness of the starting material wherethe dimension a is measured, ie at the minimum SD dimension (width) ornarrowest part of the PD strand-forming zone 11, the part that oftendetermines the strength of the product. t is taken as the “thickness ofthe starting material”, whether or not other parts of the startingmaterial are thicker.

Where a crotch edge is referred to, eg for measuring the thickness ofthe crotch edge, a point is taken as close to the crotch edge (as seenin plan view) as possible, whether or not the edge is raised or somewhattapered, though ignoring any feathering or membranous material that doesnot form part of the crotch proper, ignoring abrupt tapering requiredfor mould withdrawal in moulded starting materials, and ignoring anyradiussing caused by punching holes, though the reduction in thicknessleading up to the radiussing is not ignored; it has been found that thecrotch edge zone can be reduced in thickness by 9% to 10% as a result ofpunching alone.

The “width” is the dimension at right angles to the major axis of thezone in question, and “narrow” relates to this dimension. “Narrow” meanssignificantly longer than wide.

The stretch ratios are as measured after releasing the stretching forceor after annealing if annealing is carried out, and as measured on thesurface of the structure.

The “overall stretch ratio” is the stretch ratio applied to the wholelength of the material.

“HDPE” is high density polyethylene.

The “strength” of a mesh structure as used herein is the maximumstrength per linear unit, measured in a normal tensile test, the linearunits being in the direction at right angles to the direction in whichthe strength is measured, whether the strength is measured in the PD orin the-SD.

The “rib strength” is the maximum strength per strand.

The “economy rating” is the PD strength of the product per unit widthper mass per unit area, measured as kN per m per kg per m in the PD.

“Truth lines” are parallel lines applied (normally by printing ordrawing) to the starting material, normally in two directions parallelto the PD and SD respectively.

The “shoulder tangent” 5 is the tangent to a side of the projectingportion or protuberance 6 at the point where the tangent makes thegreatest angle with the PD, said angle being measured so that it is lessthan 90°0 if the shoulder tangent 5 is directed towards the PDcentre-line 7 of the hole 1 and towards the centre-line 8 of theadjacent SD strand-forming zone 9 and so that it is more than 90° if theshoulder tangent 5 is directed towards the PD centre-line 7 of the hole1 and away from the centre-line 8 of the adjacent SD strand-forming zone9.

The “shoulder angle” is the angle θ which the shoulder tangent 5 makeswith the PD.

The “neck tangent” 10 is the tangent to a side of the protuberance 6 atthe point where the tangent makes the smallest angle (positive ornegative) with the PD, said angle being measured so that it is positiveif the neck tangent 10 is directed towards the centre-line 8 of theadjacent SD strand-forming zone 9 and towards the PD centre-line 7 ofthe hole 1 and so that it is negative if the neck tangent 10 is directedtowards the centre-line 8 of the adjacent SD strand-forming zone 9 butgenerally away from the PD centre-line 7 of the hole 1.

The “neck angle” is the angle ψ which the neck tangent 10 makes with thePD.

The dimension “a” is the minimum SD distance between adjacentside-by-side holes 1.

The dimension “b” is the SD dimension between adjacent side-by-sideholes 1, between the points where the respective neck tangents 10 aretangent to the sides of the protuberances 6 (measured at the point wherethe neck tangent 10 is coincident with the crotch edge and furthest fromthe PD end of the hole 1, if the respective part of the protuberanceside is straight).

The dimension “c” corresponds to the dimension a, but is taken in thePD.

The dimension “d” corresponds to the dimension b, but is taken in thePD, between the points where the respective shoulder tangents 5 aretangent to the sides of the protuberances 6 (measured at the point wherethe shoulder tangent 5 is coincident with the crotch edge and furthestfrom the SD side of the hole 1, if the respective part of theprotuberance side is straight).

The “junction diagonal ratio” is the ratio of the diagonal dimension inthe biax product across two diagonally opposed points of minimum or zeroorientation of crotches and across a junction, to the same diagonaldimension i in the starting material.

The “base line” 12 of the protuberance 6 is the line tangent to each endof the protuberance 6.

The “projecting extent” j of the protuberance 6 is the greatest extentof the protuberance 6 as measured at right angles to the base line 12.

The term “soil” includes rocks, stones, gravel, sand, earth, clay,aggregate held by a binder such as asphalt, or any other particulate orcohesive material used in geotechnical engineering.

ABOUT THE INVENTION

Using the invention, it has been found that the protuberance has made itpossible to direct orientation positively through the junction,generally along the PD, while limiting the actual degree of orienationacross the centre of the junction. The invention controls and limits thelevel of orientation in the crotches whilst nonetheless maintainingcrotch edges which are oriented for at least part of their length in thedirection running around the crotch. Although the crotches can becontinuously oriented in the direction running around the crotch (ie“uniaxial” orientation), there are no crotches which are highly orientedfrom end to end.

Apart from the unoriented or low-oriented part of the crotch edge, thereis a range of orientation ratio in the crotch edge extendingcontinuously from the ratio in the unoriented or low-oriented part up tothe orientation ratio of the respective oriented strand, or to a valueeven greater than that in the oriented strand. When PD stretch isapplied, the protuberance provides a rapid increase in thecross-sectional area of the crotch-forming zone as the orientationprogresses from the yield point(s) in a PD strand-forming zone towardsthe junction; the rapid increase is preferably followed by a region ofmore slowly increasing cross-sectional area. In other words theinvention puts a block in the centre part of the crotch-forming zone,thereby preventing in a controllable manner high orientation passing allthe way around the crotch edge. The invention also controls and limitsany orientation passing behind the block, both in the PD stretch and inthe SD stretch. The block is characterised in the product by a sharpincrease in thickness (called a neck) in the crotch as one goes aroundthe crotch from the side of the respective PD strand. For instance, onecan obtain a very low crotch edge thickness reduction, of less thanabout 20% or even of less than about 5%, at the crotch-edge centreportion. As there will often be a sharp increase in thickness on eitherside of the unoriented or least oriented part of the crotch edge as thispart is approached, the crotch edge thickness should be measured at itsthickest point, or its extension should be measured at its part of leastextension; thus, if it is difficult to measure the reduction inthickness of a crotch edge, its extension (ie orientation ratio) can bemeasured, for instance by drawing parallel lines across the median planeon the side face of the starting material (which parallel lines are asclose together as practicable), and measuring their separation on thestarting material and on the product (after biaxial stretching). Theregion having the increase in thickness can have a slope of about 45° tothe plane of the mesh structure, ie an included angle of about 90°.

The effect of the invention occurs even if the PD strand-forming zonesform a large part of the SD width of the starting materal, and theinvention can be used with low SD punch-out ratios; thus the SDpunch-out can be less than about 60%, 55% or 50%, and preferably lessthan about 46%. This enables the economic manufacture of high PDstrength products from relatively low thickness starting materials.

The invention eliminates or greatly reduces any tendency to splitting oftransverse strands when the mesh structure is subject to PD forces. Inother biax products, this can occur as a result of force transmissionaround highly oriented crotches to the adjacent SD strands; suchsplitting causes failure of the mesh structure in the junction zones andprevents the strength of the oriented PD strands being fully mobilised.In other words, in the structures of the invention, the junction stresspattern is more efficient when the structure is subjected to PD forces,and the structure can have a behaviour similar to that of a uniaxstructure. The PD and SD stretches do not interact or do not interactgreatly, enabling the strength of a junction and hence the economyrating of a uniax structure to be maintained to a large extent in thefinal biax structure; thus it is preferred that during the SD stretchthe mid-point of the junction does not reduce in thickness so that theorientation of the junction mid-point is uniaxial in the PD, though thisis not essential. It is not necessary that all the SD strand-formingzone be stretched out on the SD stretch. The narrower very end portionsof the hole (considered in the PD) can provide a narrow yield point forSD orientation and provide small bar widths (PD dimension) in aconvenient manner; with suitable choice of PD pitches, the penetrationof transverse orientation into the junction during the SD stretch isreduced whilst leaving enough material to form a satisfactory junction.There are no thin zones in the junctions such as could form tearstarters and weaken the structure under the action of tensile forces.

An unexpected advantage of the invention is that the product can be veryregular, even when produced using low (slow) strain-hardening materials,ie materials with a high natural draw ratio.

It has been found that a 4 m wide biax mesh structure which appearsregular and robust to the eye can be formed from a starting material ofabout 1.5 m in width of a thickness not greater than 6.5 mm with astrength per meter width of 40 kN/m, having an acceptable failure modeand a satisfactory creep strain behaviour with adequate long termstrength. Using the invention, it is possible to achieve resultantstretch ratios of up to 18:1 or more, for instance 5.5:1 in the PD and3.3:1 in the SD. The PD stretch ratio (as measured from the mid-point ofone notional junction zone to the mid-point of the adjacent junctionzone in the PD) is preferably at least about 3:1 and more preferably atleast 4.5:1 or 5:1. For higher strength products from a given startingmaterial thickness, lower SD stretch ratios may be used, down to 1.5:1.The PD strength is significantly greater than the SD strength, eg atleast about 1.5, 2, 2.5 or 3 times the SD strength. 6.5 mm sheet issufficiently thin to be handled in production without particulardifficulties though as machinery develops in the future it isanticipated that thicker materials, eg up to 15 mm, may be readilyhandled. In one example with about 8% crotch edge thickest partthickness reduction, the short term tensile behaviour of the biaxproduct was found to be similar to that of the uniax intermediateproduct but the creep behaviour was found to be slightly worse than thatof the uniax intermediate product.

As the orientation goes right through the junction in the PD stretch,the mid-point of the notional junction zone thins down, preferably bynot more than about 10% and if possible by not more than about 5%. Dueto pulling plastics material from other parts of the structure or due torelaxation of some of the orientation inserted during the PD stretching,the junction mid-point could increase in thickness during the SDstretch. Thus the final junction mid-point thickness is measured ratherthan the uniax thickness. The junction mid-point thickness will besignificantly greater than the strand mid-point thicknesses, say atleast twice the thickness, ie a difference in thickness which is notjust due to random variation and which indicates that there is muchgreater orientation in the strand than across the junction. If desired,there can be continuous, substantially uniaxial orientation fromend-to-end of the mesh structure, which provides reasonable long-termcreep resistance, though this is not essential. In situations wherelong-term creep resistance is of less significance, one may use meshstructures of the present invention whose creep performance is inferiorto that of a good custom-designed uniax mesh structure.

It is preferred that the junction diagonal ratio be close to unity, forexample greater than about 0.8. If the ratio is unity, the diagonals ofthe junction have not altered in length during stretching, but usuallythe diagonals become shorter due to narrowing of the ends of thestrand-forming zones and the junctions caused by stretching.

In general terms, the crotch-forming zone as seen in plan view as oneprogresses towards the PD end of the hole, can have:

i) a first part which widens out;

ii) a second part which does not widen out as rapidly as the first part;

iii) a third part which widens out more rapidly than the second part andterminates the crotch-forming zone.

Said first part is preferably immediately contiguous to the second part.Said second part is preferably immediately contiguous to said thirdpart.

More specifically, the shape can be such that as one progresses towardsthe PD end of the hole:

iv) the side of said first part is progressively more inclined to the PDaxis and is defined at least in part by a curve which is concave withrespect to the hole;

v) the side of said second part is progressively less inclined to the PD(the angle of inclination can reduce to zero and then increase in theopposite sense) and is defined at least in part by a curve which is-convex with respect to the hole; and

vi) the side of said third part is progressively more inclined to the PDand is defined at least in part by a curve which is concave with respectto the hole.

The b:a ratio is preferably greater than about 1.5:1 or about 1.6:1, aspecific preferred minimum value being about 1.64:1. The higher the b:aratio, the greater the protection of the crotch and the less theorientation around the crotch. The maximum value of the b:a ratio isbelieved dictated by economical considerations (too much punch-out andlower strengths per meter width) rather than functional considerationsand may be up to about 2.5:1 or up to about 3:1 or greater.

Consideration of the b:a ratio is relevant to starting materials wherethere are protuberances in the crotch-forming zones as seen in plan viewand primarily where there is no change in thickness in thecrotch-forming zone. However, the b:a ratio can be expressed as a ratioof the cross-sectional areas in the planes corresponding to thosecontaining b and a. The cross-sectional area ratios are applicable tostarting materials where there is a change in thickness or both aprotuberance in plan view and a change in thickness. The planes would bethose immediately before the change in thickness and immediately afterthe change in thickness. In general terms, one can consider the rate ofincrease in SD cross-sectional area over a certain PD distance.

At least up to 90°, the greater the shoulder angle θ, the greater theprotection of the crotch and the less orientation around the crotch. Anyinflection or protuberance enables a satisfactory product to bemanufactured at reduced SD punch-out, and it is believed that the closerthe protuberance is to having right-angled shoulders, the lower the SDpunch-out that may be used if a certain mode of failure is preserved.The angle θ is preferably more than about 50°, 60°, 65°, 70° or 75°; theangle θ is preferably less than about 135°, 125°, 115°, 110° or 105°;the preferred angle θ is about 90°.

The angle χ between the shoulder tangent 5 and the neck tangent 10 ispreferably roughly 90° or an obtuse angle. The neck angle ψ ispreferably about 0°. However, it is not excluded that the neck angleshould have a negative value, ie with the hole 1 widening out again froma narrower “neck” as one approaches the PD end of the hole 1.

Considered in the SD, the crotch-forming zones can be as (i) to (iii) or(iv) to (vi) as set out above in relation to the crotch-forming zones asconsidered in the PD. The d:c ratio determines at least in part theamount the junction and crotch centre are thinned and oriented in the SDstretch. The d:c ratio is preferably greater than about 1.5:1 or greaterthan about 2:1; the ratio is preferably less than about 3:1 or less thanabout 4:1. By having the d:c ratio sufficiently large, any reduction ofthe thickness of the centre of the junction during the SD stretch can beavoided. In order to prevent the SD orientation penetrating too fartowards the mid-point of the junction, the width c of the SDstrand-forming zone 9 can be reduced or the bar thickness can bereduced, for instance by extruding material with PD grooves registeringwith the centre parts of the holes or for instance using the techniquedescribed in U.S. Pat. No. 4,590,029. The SD orientation can be takenbeyond the line on which the dimension d is measured. It is preferredthat during the SD stretch, the orientation does not extend so fararound the crotch that the whole of the crotch is oriented.

The base line 12 of the protuberance 6 preferably makes an angle φ withthe PD of mnore than about 30° or 35°; the angle φ is preferably lessthan about 60° or 55°. The projecting extent j can be about 3% to about15%, preferably about 5% to about 10%, of the length of the diagonal i.If the base line 12 of the protuberance 6 is too short and if theprojecting extent j is too great, there will be a tendency for theorientation to go behind the protuberance 6 as though the protuberance 6were not there.

Preferably, the very LD ends of the holes 1 will be formed by continuouscurves, though this is not necessarily so as the ends could be formed bystraight SD lines. The sides of the crotch-forming zones 13 will beformed bycurves (which are not necessarily circular arcs) which canconnect directly into one another or can be connected with one anotherby straight lines; the connections are smooth, ie no very small radiuskinks or notches, at least where there is a connection that is concaveto the hole 1. The radii should not be so small as to cause a notch andnot so large as to impair the blocking function of the protuberance. Theradii chosen can have a significant effect on the formation of theblock.

Normally, it is preferred that each hole 1 be symmetrical about the PDaxis and/or about the SD axis. The central parts of the sides of the PDstrand-forming zones, ie the parts connecting said PD end portions, canbe any suitable shape, eg of “diabolo” shape as disclosed in U.S. Pat.No. 4,743,486, straight, or elliptical concave with respect to the hole1. Grooving techniques such as the cold grooving described in U.S. Pat.No. 4,590,029 can be applied to the PD or SD strand-forming zones 11, 9.

The area of the PD strand-forming zone, as seen in SD section normal tothe mesh structure between the respective PD tangent lines, ispreferably less than about twice the area of the SD strand-forming zoneas seen in section normal to the mesh structure and along the axis ofthe centre line of the SD strand-forming zone from the PD tangent lineat one end of the zone to the PD tangent line at the other end of thezone. On a flat sheet, this ratio is equivalent to the SD punch-out.

Complex Starting Material Hole Configurations

The invention is applicable to more complex starting materials of squareconstruction where some of the holes are different and are not on thesame notional square or rectangle grid as the shaped holes of theinventions, eg as described in U.S. Pat. Nos. 4,536,429 or 4,574,100.However, in general, the invention is either applied to substantiallyevery junction, or, with said more complex starting materials (wherethere will be junctions of different types in the biax products), theinvention is applied to substantially every junction of a certain type.

Depending upon the configuration of the holes in the starting material,a plurality of junctions could be connected tog ether not by orientedstrands but by unoriented or low-oriented material. In this manner,there can be a plurality of junctions connected in the PD and/or in theSD, the so-connected junctions interconnecting six or more orientedstrands. The junction mid-point con tinues to be that point thatcorresponds to the mid-point of the notional junction zone, taking allholes into account unless they do not contribute to the formation of anoriented strand. Thus the invention is applicable to just one corner ofa junction; each crotch which interconnects an oriented PD strand and anoriented SD strand can be of the form of the invention with the othercrotches being of other forms. With such a mixed starting material, thedimensions a and b or c and d can be measured to the centre line of therespective PD or SD strand-forming zone.

In one form of starting material that provides such connected junctions,there are holes in accordance with the invention aligned in the PD; inthe SD, holes in accordance with the invention are interspersed betweenother holes which are not in accordance with the invention, eg holes asdisclosed in U.S. Pat. No. 4,743,486. In the SD, there can be one saidother hole between hoies in accordance with the invention, or, if lessSD extension is required, two, three or any suitable number of saidother holes. The PD distance between said other holes may need to begreater than the PD distance between the holes in accordance with theinvention. The holes are such that during the SD stretch, only the SDstrand-forming zones between the holes in accordance with the inventionstretch out. It is found that such a structure has better long-termcreep performance, though its SD dimension will not be maximised,overall SD stretch ratios being reduced to say about 1.5:1 or less.

In another form of the invention, holes of the invention areinterspersed in the PD between other holes which are not in accordancewith the invention, the SD width of the latter other holes may need tobe less than that of the holes. of the invention, ie in the PD, theremay be wider holes (greater SD dimension) interspersed with narrowerholes. The narrower holes between the holes of the invention can assistin producing the crotch block effect of the invention. On stretching inthe PD, the narrow gaps between the PD ends of the wider and narrowerholes prevent the crotch being stretched right out and assist in theformation of a block; on stretching in the SD, the narrow gaps act asnarrow yield points and assist in preventing large forces being appliedacross the junction.

In another form of starting material, the invention is applied to oneside of the hole but not to the other side of the hole, giving a holewhich is asymmetrical about its PD axis.

There may be other ways of achieving the crotch block effect.

There is available a mesh structure in which the SD oriented strands areeach wholly or partially divided into two generally superimposed strandswhich may be somewhat displaced relative to each other in the PD,looking at the structure in plan, formed from an extruded startingmaterial in which a slot is formed in the PD through the middle of theSD strand-forming zone. The present invention could be applied to such astarting material. Structures produced in this manner are expected tohave the advantages of the invention.

Starting Materials

The starting materials can be in general as set out from column 10,lines 20 to 24 and from column 10, line 32 to column 11, line 6 of U.S.Pat. No. 5,269,631, which disclosure is incorporated herein byreference. The preferred plastics materials are polyolefins. Theexperimental work on which the present invention is based was performedon medium molecular weight HDPE, which has a relatively low rate of workhardening. Other material, and also factors such as different stretchingtemperatures and rates and different degrees of restraint at rightangles to the stretching direction, can significantly change the meshstructure produced. Thus, test pieces of the starting material should bestretched on a laboratory scale to determine whether they perform inaccordance with the invention.

The holes 1 can be made in any suitable way; the preferred way is topunch, though moulding is possible and in theory a Hureau-type orRical-type or Sire-type die head (as in FR 2 131 842 or U.S. Pat. No.3,252,181) could be used if the hole shape and thickness profiles can besufficiently well defined. The starting materials need not be strictlyor even substantially flat and can be for instance as disclosed in U.S.Pat. No. 5,053,264 or the bumpy materials which may be formed byextrusion with the Hureau-type or Rical-type or Sire-type die head.Normally, a moulding process and a Hureau-type or Rical-type orSire-type die head provide holes with tapered sides. The startingmaterials need not be substantially uniplanar though are preferably so.The starting materials may be grooved as described above or may bespecifically profiled, eg as described in U.S. Pat. No. 5,267,816. Thestarting materials are preferably not substantially oriented, thoughmelt flow orientation can be present.

Testing

As the behaviour of the starting material varies with many factors suchas the resin used, the thickness, the stretching temperature, the holeshapes and the hole pitches, laboratory samples should be produced andtested before carrying out tests on a production line, to ascertainwhether the desired orientation behaviour is achieved. The applicationof truth lines to a laboratory sample greatly assists in observing theorientation behaviour.

Production Plant

Though not essential, the PD stretch will normally precede the SDstretch. In production, the PD will often be the machine direction (MD),but it could be the transverse direction (TD). Although it is normallymore economical to carry out sequential MD/TD stretching with the TDstretch following the MD stretch, it is not essential for the presentinvention that the TD stretch should follow the MD stretch. Thus the PDstretch can be carried out before or after the SD stretch. Furthermore,multi-stage stetching can be employed, particularly in the PD; in such acase, the PD orientation need not be taken right through the junctionuntil the final PD stretch. In normal manufacturing practice, the meshstructure is formed as a long MD length which is rolled up. It is notnecessary to cool between stretches, and all or any two or any threestretching operations can be carried out in-line without cooling.

The plant illustrated in FIG. 11 or 11 a of U.S. Pat. No. 4,374,798 canbe used in commercial production to carry out the stretching operationsof the inventions in two stages; alternatively the stretching may be inthree or more stages, eg as described in U.S. Pat. No. 5,269,631.

Uses

The principal use is for strengthening soil and making a geotechnicalstructure, as referred to above, when the biaxially-stretched structurewould be referred to as a geogrid. The uses set out from column 17,lines 45 to 61 and from column 17, line 64 to column 18, line 59 of U.S.Pat. No. 5,269,631 are appropriate for the mesh structures of thepresent invention. The mesh structures of the present invention areparticularly suitable for retaining walls or embankments, with the PDgenerally at right angles to the face of the wall or embankment as seenfrom above. However, the mesh structures can be used for other purposes.For instance, the mesh structures can be used in asphalt roadconstructions, particularly to avoid reflective cracking, or can be usedin concrete constructions in order to control shrinkage cracking.

PREFERRED EMBODIMENTS

The invention will be further described, by way of example withreference to the accompanying drawings, in which:

FIG. 1a is a diagram of four holes in a first starting material formaking a geogrid;

FIG. 1b is a plan view of the same starting material as in FIG. 1a, butshowing truth lines;

FIG. 1c is a plan view of the uniax intermediate material made from thestarting material of FIG. 1b;

FIG. 1d is a plan view of the intermediate material of FIG. 1c, showingthe thicknesses;

FIG. 1e is a plan view of the biax product made from the startingmaterial of FIG. 1b;

FIG. 1f is a plan view of one of the junctions shown in FIG. 1e, showingthe thicknesses;

FIG. 1g shows the dimensions of the holes in the starting material ofFIG. 1a;

FIGS. 2 to 6 show the dimensions of holes of second to sixth startingmaterials;

FIGS. 7a to 7 e correspond to FIGS. 1b, 1 c and 1 d to 1 f but using aseventh starting material, the holes being as in FIGS. 1g and 7 f;

FIGS. 8a to 8 e correspond to FIGS. 1a to 1 e but using an eighthstarting material, the larger holes being as in FIG. 1g and the smallerholes being 4.763 mm long with 1.588 mm radius semi-circles at each end;

FIG. 9a is a plan view of a first grooved starting material;

FIG. 9b is a section along the line IXB—IXB in FIG. 9a;

FIGS. 10a and 10 b correspond to FIGS. 9a and 9 b, and show a secondgrooved starting material; and

FIGS. 11 to 15 are vertical sections through different geotechnicalconstructions incorporating the mesh structure of the invention.

The truth lines shown in the Figures can be screen-printed on thestarting material, and indicate the progress of orientation, at least onthe surface of the structure. They would not normally be provided oncommercial materials.

The contour hatching in the Figures indicates a steep slope (increase inthickness), the hatching lines extending up the slope.

When more than one like junction of a stretched material is shown, alljunctions are shown as identical although in production there could besmall differences between junctions. In Figures which show successivestages, the junctions are not necessarily the same junction and thetruth lines may be in somewhat different positions as it was necessaryto use different test pieces.

In the Figures all dimensions are given in millimeters; wherethicknesses are given, they are the thicknesses at the points indicated.A dimension with a single arrow indicates a circular arc. All arcs arecircular arcs.

FIG. 1 a

FIG. 1a shows various items defined above. FIG. 1a also showscrotch-forming zones 13 adjacent the corners of the notional junctionzones 4, each lying between a respective hole 1, the adjacent PD tangentline 3 and the adjacent SD tangent line 2, one such zone 13 being showncross-hatched in. If the holes 1 terminate in a rectilinear portionextending in the SD, the crotch-forming zones 13 will not define thevery PD ends of the holes 1. In FIG. 1a, each PD strand-forming zone 11can be considered as a zone between respective holes 1 and at its endsbetween respective crotch-forming zones 13, which extends from the SDtangent line 2 tangent to the first PD ends of the respective holes 1 tothe SD tangent line 2 tangent to the other, second PD ends of the holes1, one such zone 11 being shown cross-hatched. In FIG. 1a, each SDstrand-forming zone 9 can be considered as the zone between respectiveholes 1 and between respective crotch-forming zones 13, which extendsfrom the PD tangent line 3 tangent to the first SD ends of therespective holes 1 to the PD tangent line 3 tangent to the other, secondSD ends of the holes 1, one such zone 9 being shown cross-hatched.

FIGS. 1 e to 1 f

The starting material 21 in FIG. 1b has been punched from strictly flatsheet. The hole 1 shape and dimensions are as shown in FIGS. 1a and 1 g.The hole 1 is of waisted shape, in accordance with U.S. Pat. No.4,743,486. The thickness can be 6 mm, and details can be as in Example 1of the Table below. In the manner indicated with reference to FIG. 1a,the crotch-forming zones 13 have protuberances 6 with features (i) to(vi) referred to above. The yield points of the PD strand-forming zones11 are at their narrowest parts (where the dimension a is shown in FIG.1a).

FIGS. 1e and 1 d show the uniax intermediate material 22, after theapplication of PD stretch. The PD strand-forming zones 11 have beenstretched out into highly oriented PD strands 23 interconnected bytransverse bars 24. As can be seen from slight splaying of the SD truthlines, a small amount of PD orientation has been taken right through thenotional junction zones 4 and there is continuous uniaxial orientationextending from end to end of the structure.

From the thickness chart of FIG. 1d, it can be seen that the PDorientation in the PD strands 23 has extended round the crotches, butthe size of the protuberances 6 is such that they have acted as a blockand the orientation ratio decreases significantly as the orientationapproaches the central part of the crotch edge, although the decreasemay differ slightly from crotch to crotch. For each crotch, theorientation ceases just beyond the centre of the crotch edge.

FIGS. 1e and 1 f show the biax product or geogrid 25 after theapplication of SD stretch. The SD stretch is such that the SDorientation ceases just before the crotch edge centres, therebyproviding continuous orientation around the crotches from end to end inthe direction of the crotches, but with low orientation in the crotchedge centres, the crotch edge centre thickness reductions noted in theuniax intermediate material 22 remaining substantially the same. Themaximum thickness of the thickest part of the crotch edge does notcoincide with the centre of the protruberance 6, and is more than about80% of the thickness of the mid-point of the biax junction 27. The SDorientation is not taken across the centre of the notional junction zone4, ie there is no further reduction in thickness of the centre of thenotional junction zone 4, and the junction mid-point remains uniaxiallyoriented; the SD orientation ends at about 1.5 mm from the junctionmid-point. FIG. 1f is G somewhat anomalous because some of thethicknesses are greater than in FIG. 1d, due to using different samples.The central portions of the SD strand-forming zones 9 have beenstretched out into highly oriented SD strands 26. The junction 27comprises a generally rectangular central zone 28 which has a slight diprunning through its middle in the PD. The central zone 28 is howeverthicker than zones 29 on all four sides of the central zone 28; thezones 29 on the SD sides and/or PD sides may not be in the not ionaljunction zone. There are narrow projections 30 at the corners of thecentral zone 28, extending outwards, continuing through the crotches,and running into the crotch edges. The central parts of the crotch edgesare convex relative to the junction. FIG. 1f indicates with dashed lines31 the notional lines (ridge lines) of maximum thickness from themid-point of the junction 27 to the crotch edge.

The PD strands 23 are closer together than the SD strands 26, althoughthey need not be so, and have a greater cross-sectional area than the SDstrands 26, although they need not have, but one of these featuresshould be present so that the biax product 25 has significantly greaterstrength in the PD than in the SD.

FIGS. 2 to 6

These Figures show the shape and dimensions of the holes 1 for thesecond to sixth starting materials. The holes 1 shown in FIGS. 2, 4, 5and 6 are of waisted shape, in accordance with U.S. Pat. No. 4,743,486.The hole shape shown in FIG. 4 is the same as that of FIG. 1g, exceptthat the end portions have a zone with parallel sides and the very endradius is relatively large.

FIGS. 7 a to 7 f

These Figures will not be described in detail. In the starting materialmain holes 1 in accordance with the invention are interspersed in the SDbetween other, subsidiary holes 32 which are not in accordance with theinvention and are shorter in the PD than in the main holes 1. In the PDstretch, all the PD strand-forming zones stretch out. In the SD stretch,the wide SD strand-forming zones between the holes 32 do not stretch outand remain unoriented or only slightly oriented. In the biax product,pairs of junctions (notional junction zones) are connected in the SD bya zone of low-oriented material as shown in FIG. 7e. However, for theleft and right hand crotches of the junction pairs, the orientationbehaviour is generally as in FIGS. 1e and 1 f. The top and bottom middlecrotches of the junction pair are not oriented. The mid-points of thenotional junction zones are indicated by the thicknesses 5.83 and 5.85in FIG. 7e. The ridge lines 31 are taken from these mid-points, and areshown running into the left and right hand crotches of the junctionpair. In the biax product 25, the PD strands 23 are in groups of two.

FIGS. 8 a to 8 e

These Figures will not be described in detail. In the starting material,main holes 1 in accordance with the invention are interspersed in the PDbetween other, subsidiary holes 33 which are not in accordance with theinvention and are narrower than the holes 1. In the PD stretch, the PDstrand-forming zones between the holes 1 stretch out but the wider zonesbetween the holes 33 do not stretch out significantly; some minorcontraction occurs in the SD. In the SD stretch, all the SDstrand-forming zones stretch out. In the biax product, pairs ofjunctions (notional junction zones) are connected in the PD by a zone oflow-oriented material whose mid-point is indicated by the thickness 5.97in FIG. 8e and is biaxially oriented. However, for the top and bottomcrotches of the junction pair, the orientation behaviour is generally asin FIGS. 1e and 1 f, although there is some biaxial orientation in thecentres of the notional junction zones. The crotches in the middle ofthe sides of the junction pair are unoriented or only slightly oriented.Ridge lines 31 are shown running from the notional junction zonemid-points into the top and bottom crotches of the junction pair.

FIGS. 9 a and 9 b

FIGS. 9a and 9 b show a first starting material 21 which has deep PDgrooves 41 with tapered sides passing through the centres of the holes1. It is believed that different dimensional parameters, including theb:a and d:c ratios will apply for such starting materials than theparameters described herein for flat starting materials. In FIG. 9a, asone progresses from the PD strand-forming zone around the crotch edge,there is initially no decrease in thickness and then a rapid decrease inthickness at the site of the protuberance 6. The grooves 41 provide goodyield points for the SD stretch and the rapid increase in thickness asone progresses along the SD strand-forming zone prevents excessivepenetration of the SD orientation into the junction. In the product, thethickest part of the crotch edge is at a position corresponding to aposition on the starting material just before the rapid decrease inthickness.

FIGS. 10 a and 10 b

FIGS. 10a and 10 b correspond closely to FIGS. 9a and 9 b, except thatthe grooves 29 are narrower.

EXAMPLES

A number of laboratory samples were prepared using flat HDPE sheet ofabout 0.95 g/cc density and nominal thickness 6.0 mm. The sheet ofExample 2 was made from a Soltex resin produced by Solvay; the sheet forthe other Examples was made from a resin produced by Borealis. Thelatter resin is slightly poorer when the biax products are compared butwas available for large scale manufacture. Both resins were mediumstrain-hardening material. Sheets were punched and screen printed with a1.6 mm square grid pattern. The sheets were stretched at a temperatureof 105° C. in two directions at right angles at a relative cross-headspeed of 250 mm/min, being stretched sequentially first in the PD andthen in the SD. The samples were effectively unrestrained in the SDduring the PD stretch; the samples were all restrained in the PD duringthe SD stretch to simulate manufacture in a plant with PD stretchingfirst in the MD, the restraint being close to 100%. The stretch ratiosare overall (after any contraction that may occur after releasing thestretching force). Test specimens were cut from the samples, eachcomprising a single strand with three junctions except for Example 6where the specimens comprised pairs of PD strands. For tensile testing,each specimen was gripped by jaws at the two extreme junctions, themiddle junction being centrally disposed. Tensile loads were appliedaxially (PD or SD) to the specimen at a cross-head displacement rate of50 mm/min. All testing was at 20° C.±1° C.

Examples and test results are given in the following Table. “n/a”indicates that the relevant measurement test or calculation was notcarried out. In Example 7, the SD punch-out is given for the main(larger) holes and for the sub (smaller) holes. In the headings, “uniax”means the intermediate structure after the PD stretch, “biax” means thefinal product. The crotch edge thicknesses are the average thicknessesfor a single junction measured. Though there are connected junctions inthe biax products of Examples 6 and 7, the biax junction mid-pointthickness is expressed as the average thickness of the mid-points of thetwo junctions.

TABLE Example Hole shape PD pitch SD pitch SD punch-out, % Projectingextent (j), mm b:a d:c θ ψ χ φ 1 FIG. 1g 38 24 51 1.5 1.6:1 2.0:1 90 090 39 2 FIG. 1g 38 27 45 1.5 1.5:1 2.0:1 90 0 90 39 3 FIG. 2 38 24 511.0 1.6:1 2.0:1 67 0 113 33 4 FIG. 3 38 24 53 1.5 1.5:1 2.0:1 90 0 90 395 FIG. 4 38 24 51 1.8 1.6:1 2.0:1 90 0 90 34 6 FIG. 7a 38 24 50 1.51.6:1 2.0:1 90 0 90 39 7 FIG. 8a 46 27 45 main 1.5 1.5:1 1.8:1 90 0 9039 13 sub Distance of Starting Thickness SD Uniax junction Uniax %material of crotch PD SD orientation Junction mid-point reduction inthickness edge after stretch stretch from junction diagonal thickness,junction mid- Example a:t c:t (t), mm punching, mm ratio ratiomid-point, mm ratio mm point thickness 1 2.0:1 1.6:1 6.1 5.9 5.0:1 2.7:16 0.9:1 5.9 4.1 2 2.5:1 1.6:1 6.0 5.5 5.0:1 2.7:1 7 0.9:1 5.7 4.2 32.0:1 1.6:1 6.1 6.0 5.0:1 2.7:1 5 0.9:1 5.8 4.4 4 2.0:1 1.6:1 6.1 6.05.0:1 2.7:1 6 0.9:1 5.8 5.2 5 2.0:1 1.6:1 6.1 6.0 5.0:1 2.7:1 5 0.9:15.9 3.6 6 2.0:1 1.6:1 6.1 <6.1 5.0:1 1.8:1 n/a 1.0:1 5.9 3.6 7 2.5:12.1:1 6.1 <6.1 3.9:1 2.6:1 n/a 1.0:1 n/a 0.8 Biax % reduction Ratio biaxBiax % reduction Biax thickness in thickness thickest crotch Biaxjunction in junction mid- of thickest of thickest Biax ridge-line Biaxridge-line edge part: mid-point point crotch edge crotch edge minimumthickness ratio, junction mid- Example thickness, mm thickness part partthickness, mm % point, % 1 5.8 4.4 5.3 9.8 5.2 90 91 2 5.7 3.7 5.4 3.15.4 94 94 3 5.8 4.3 5.0 17 5.0 87 85 4 5.9 3.9 5.5 8.4 5.3 90 93 5 5.94.1 5.4 9.7 5.3 89 92 6 5.8 4.9 5.3 11 5.3 89 89 7 5.9 3.3 5.5 8.8 5.592 92 Biax PD Uniax PD Biax Biax PD economy Biax SD Biax Biax peak UniaxPD peak peak Biax PD Biax rating, Biax PD peak SD PD/SD Biax load,rupture stress, load, strength, weight, kN/m/ rupture load, strength,rib PD/SD Example kN/rib mode*** MPa kN/rib kN/m g/m² kg/m² mode***kN/rib kN/m strength strength 1 2.58 1 266 2.61 40.8 298 137 1 2.00 10.51.3 3.9 2 2.92 1 242 2.93 40.0 309 129 1 n/a n/a n/a n/a 3 2.71 1 2682.55 39.2 293 134 3 1.99 10.4 1.3 3.8 4 2.67 1 306 2.60 40.6 296 137 31.98 10.3 1.3 3.9 5 2.59 1 257 2.53 39.0 290 134 2 1.84  9.6 1.4 4.1 62.611 1 243 2.60 61.0 467 131 1 & 4 1.90 10.0 1.4 6.1 7 3.092 1 214 3.0444.0 438 101 1 2.86 15.9 1.1 2.8 ***1 All strand breaks 2 Mostly strandbreaks 3 Mostly ductile junction failures 4 All ductile junctionfailures

FIGS. 11 to 15

FIGS. 11 to 15 are schematic illustrations of known geotechnicalstructures, in which geogrids 25 in accordance with the invention areused in place of earlier geogrids. In each case, the geogrid 25 is shownwith its PD extending across the plane of the paper and its SD extendingout of the plane of the paper. For short lengths of geogrid 25, themanufacturing technique could be such that the geogrid has a muchgreater dimension in the SD than in the PD so that a single width ofgeogrid extends across a broad part of the face of the geoconstruction.

BRIEFLY

FIG. 11 shows a retained wall 51 formed of full-height panels which canbe vertical or inclined by up to 45° to the vertical. The geogrids 25are in parallel layers and fixed to the wall 51.

FIG. 12 shows a retained wall 52 formed of discrete blocks orincremental panels, which wall 51 can be vertical or inclined by up to45° to the vertical. The geogrids 25 are fixed to the blocks or panelsor are securDed between or to them in any suitable manner, eg byfriction or using mortar or pins or special fixings.

FIG. 13 shows the face of a retained embankment 52 with the geogrids 25in parallel layers and the end of each geogrid 25 taken up at the faceand brought back into the soil, the brought-back part 26 being incontact with the geogrid 25 of the next layer up.

FIG. 14 shows a simpler retained embankment 52 construction, with thegeogrids 25 terminating at the face.

FIG. 15 shows an embankment 52 whose base is stabilised by a geogrid 25.There may be multiple, vertically-spaced layers of geogrid 25 in anysuitable arrangement, above the geogrid 25 shown.

The disclosures of the patent specifications referred to above areincluded in this disclosure by reference.

The present invention has been described above purely by way of example,and modifications can be made within the spirit of the invention.

What is claimed is:
 1. A method of producing a biaxially-stretchedplastics material geogrid for use in a geotechnical construction, havinga greater strength in a primary direction (PD) than in a secondarydirection (SD) substantially at right angles to the PD, comprising:providing a substantially uniplanar plastics starting material which hasa thickness of at least about 2 mm and has a pattern of through-holes ona substantially square or rectangular grid whose axes are substantiallyparallel to the PD and to the SD respectively, the sides of at leastsome PD end portions of said holes being defined by crotch-forming zoneshaving protuberances; applying PD stretch to form oriented PD strandsand to apply some orientation to the junction-forming zones so thatorientation extends into and through the junction- forming zones; andapplying SD stretch with an overall stretch ratio of at least about1.5:1 as measured from the mid-point of one junction-forming zone to themid-point of the adjacent junction-forming zone in the SD to formoriented SD strands; in the geogrid so produced, the mid-point of thejunction zone being substantially thicker than the mid-point of anyoriented strand entering the junction zone, and the edge of the crotchinterconnecting adjacent sides of adjacent oriented PD and oriented SDstrands being oriented in the direction running around the crotch withthe orientation ratio decreasing significantly as one passes around thecrotch edge either from the oriented PD strand or from the oriented SDstrand, the crotch edge either a) having an unoriented part, or b) thethickness of the least oriented part of the crotch edge being reduced,or the length of the least oriented part of the crotch edge beingincreased, by no more than about 20% by the action of stretching, andthe action of stretching not reducing the thickness of any point alonggenerally diagonally-extending lines of maximum thickness on thebiaxially-stretched mesh structure from the mid-point of the junctionzone to said crotch edges to such an extent that the ratio of finishedthickness to starting thickness at that point is less than about 80% ofthe ratio of finished thickness to starting thickness of the junctionzone mid-point.
 2. The method of claim 1, wherein the protuberances arein the plan view shape of the hole.
 3. The method of claim 2, wherein,as seen in plan view as one progresses towards the PD end of the hole,each said crotch-forming zone has: i) a first part which widens out; ii)a second part which does not widen out as rapidly as the first part; andiii) a third part which widens out more rapidly than the second part andterminates the crotch-forming zone.
 4. The method of claim 1, whereinthe ratio of the SD dimension between adjacent side-by-side holesbetween respective points where the respective tangent to a side of theprotuberance makes the smallest angle with the PD or between the pointswhere the tangent is coincident with the crotch edge and furthest fromthe PD end of the hole if the respective part of the protuberance sideis straight, with respect to the minimum SD distance between adjacentside-by-side holes is greater than about 1.5:1.
 5. The method of claim1, wherein the ratio of the PD dimension between adjacent side-by-sideholes between respective points where the respective tangent to a sideof the protuberance makes the greatest angle with the PD or between thepoints where the tangent is coincident with the crotch edge and furthestfrom the SD side of the hole if the respective part of the protuberanceis straight, with respect to the minimum PD distance between adjacentside-by-side holes is greater than about 1.5:1.
 6. The method of claim1, wherein the PD stretch is applied with an overall stretch ratio of atleast about 3:1 as measured from the mid-point of one junction-formingzone to the mid-point of the adjacent junction-forming zone in the PD.7. The method of claim 1, wherein the SD punch-out is less than about60%.
 8. The method of claim 1, wherein said plastics material is apolyolefin.
 9. The method of claim 1, wherein holes having at least onecrotch-forming zone as defined in claim 1 are interspersed in the SDbetween other holes which do not have a crotch-forming zone as definedin claim
 1. 10. The method of claim 1, wherein holes having at least onecrotch-forming zone as defined in claim 1 are interspersed in the PDbetween other holes which do not have a crotch-forming zone as definedin claim
 1. 11. A biaxially-stretched integral plastics material meshstructure produced by the method of claim
 1. 12. A biaxially-stretchedsubstantially uniplanar integral plastics material geogrid for use in ageotechnical construction having a greater strength in a primarydirection (PD) than in a secondary direction (SD) substantially at rightangles to the PD and having a first face and a second face, made from astarting material which has a thickness of at least about 2 mm, andcomprising oriented PD strands and oriented SD strands, interconnectedby junctions whose mid-point thickness is substantially greater than themid-point of any oriented strand entering the junction, adjacent sidesof adjacent oriented PD and oriented SD strands being interconnected bycrotches whose edges for at least part of their length are oriented withorientation running in the direction around the crotch, said PD and SDstrands and said crotches defining mesh openings substantially free offibrils, the orientation ofthe PD strands extending into and through thejunctions, and the junctions comprising a central zone which is thickerthan thinner zones in the junction adjacent the ends of oriented PD andoriented SD strands, the junction central zone having on each facethereof a narrow generally diagonally-extending projection at the cornerextending outwards between said thinner zones, continuing through thecrotch between the oriented PD and oriented SD strands and running intothe crotch edge, with no point on a line of maximum thickness on thegeogrid from the junction mid-point to the crotch edge having athickness of less than about 80% of the thickness of the junctionmid-point.
 13. The geogrid of claim 12, wherein said junctioninterconnects two oriented PD strands entering the junction fromopposite sides and two oriented SD strands approaching or entering thejunction from two other opposite sides, and the junction central zone issubstantially square or rectangular with a said narrow projection ateach corner.
 14. The geogrid of claim 12, wherein saidjunction isconnected by non-oriented or low-oriented material to an adjacentjunction in the SD.
 15. The geogrid of claim 12, wherein said junctionis connected by non-oriented or low-oriented material to an adjacentjunction in the PD.
 16. A geotechnical construction, comprising a massof particulate material strengthened by embedding therein the geogrid ofclaim 11, said geogrid reinforcing said mass whereby said mass createstensile forces in said PD strands and said geogrid substantiallymaintains the configuration of said mass.
 17. A method of producing abiaxially-stretched plastics material geogrid for use in a geotechnicalconstruction, having a greater strength in a primary direction (PD) thanin a secondary direction (SD) substantially at right angles to the PD,comprising: providing a substantially uniplanar plastics startingmaterial which has a thickness of at least about 2 mm and has a patternof through-holes on a substantially square or rectangular grid whoseaxes are substantially parallel to the PD and to the SD respectively,the sides of at least some PD end portions of said holes being definedby crotch-forming zones in which, as seen in plan view as one progressestowards the PD end of the hole, each said crotch-forming zone has: (i) afirst part having a side which is progressively more inclined to the PDand is defined at least in part by a curve which is concave with respectto the hole; (ii) a sccond part having a side which is progressivelyless inclined to the PD and is defined at least in part by a curve whichis convex with respect to the hole; and (iii) a third part having a sidewhich is progressively more inclined to the PD and is defined at leastin part by a curve which is concave with respect to the hole; applyingPD stretch to form oriented PD strands and to apply some orientation tothe junction-formning zones so that orientation extends into and throughthe junction-forming zones; and applying SD stretch with an overallstretch ratio of at least about 1.5:1 as measured from the mid-point ofone junction-forming zone to the mid-point of the adjacentjunction-forming zone in the SD, to form oriented SD strands; in thegeogrid so produced, the mid-point of the junction zone beingsubstantially thicker than the mid-point of any oriented strand enteringthe junction zone, and the edge of the length of the crotchinterconnecting adjacent sides of adjacent oriented PD and oriented SDstrands being oriented in the direction running around the crotch withthe orientation ratio decreasing significantly as one passes around thecrotch edge either from the oriented PD strand or from the oriented SDstrand, the crotch edge either a) having an unoriented part, or b) thethickness of the least oriented part of the crotch edge being reduced,or the length of the least oriented part of the crotch edge beingincreased, by no more than about 20% by the action of stretching, andthe action of stretching not reducing the thickness of any point alonggenerally diagonally-extending lines of maximum thickness on thebiaxially-stretched mesh structure from the mid-point of the junctionzone to the crotch edges to such an extent that the ratio of finishedthickness to starting thickness at that point is less than about 80% ofthe ratio of finished thickness to starting thickness of the junctionzone mid-point.
 18. A method of producing a biaxially-stretched plasticsmaterial geogrid for use in a geotechnical construction, having agreater strength in a primary direction (PD) than in a secondarydirection (SD) substantially at right angles to the PD, comprising:providing a substantially uniplanar plastics starting material which hasa thickness of at least about 2 mm and has a pattern of through-holes ona substantially square or rectangular grid whose axes are substantiallyparallel to the PD and to the SD respectively, the sides of at leastsome PD end portions of said holes being defined by crotch-forming zonesin which, as seen in plan view as one progresses towards the PD end ofthe hole, each said crotch-forming zone has: (i) a first part having aside which is progressively more inclined to the PD axis and is definedat least in part by a curve which is concave with respect to the hole;(ii) a second part having a side which is progressively less inclined tothe PD and is defined at least in part by a curve which is convex withrespect to the hole; and (iii) a third part having a side which isprogressively more inclined to the PD and is defined at least in part bya curve which is concave with respect to the hole; applying PD stretchto form oriented PD strands and to apply some orientation to thejunction-formning zones so that orientation extends into and through thejunction-forming zones; and applying SD stretch with an overall stretchratio of at least about 1.5:1 as measured from the nmid-point of onjunction-forming zone to the mid-point of the adjacent junction-formingzone in the SD, to form orientated SD strands; in the geogrid soproduced, the mid-point of the junction zone being substantially thickerthan the mid-point of any oriented strand entering the junction zone,and the edge of the length of the crotch interconnecting adjacent sidesof adjacent oriented PD and oriented SD strands being oriented in thedirection running around the crotch with the orientation ratiodecreasing significantly as one passes around the crotch edge eitherfrom the oriented PD strand or from the oriented SD strand, the crotchedge either a) having an unoriented part, or b) the thickness of theleast oriented part of the crotch edge being reduced, or the length ofthe least oriented part of the crotch edge increased, by not more thanabout 20% by the action of the stretching.
 19. The method of claim 17,wherein the SD punch-out is less than about 60%.
 20. A method ofproducing a biaxially-stretched plastics material geogrid for use in ageotechnical construction having a greater strength in a primarydirection (PD) than in a secondary direction (SD) substantially at rightangles to the PD, comprising: providing a substantially uniplanarplastics starting material which has a thickness of at least about 2 mmand has a pattern of through-holes on a substantially square orrectangular grid whose axes are substantially parallel to the PD and tothe SD respectively, and wherein the SD punch-out is less than about60%, the sides of at least some PD end portions of said holes beingdefined by crotch-forming zones in which, as seen in plan view as oneprogresses towards the PD end of the hole, each said crotch-forming zonehas: (i) a first part having a side which is progressively more inclinedto the PD axis and is defined at least in part by a curve which isconcave with respect to the hole; (ii) a second part having a side whichis progressively less inclined to the PD and is defined at least in partby a curve which is convex with respect to the hole; and (iii) a thirdpart having a side which is progressively more inclined to the PD and isdefined at least in part by a curve which is concave with respect to thehole; applying PD stretch to form oriented PD strands and to apply someorientation to the junction-forming zones so that orientation extendsinto and through the junction-forming zones; and applying SD stretchwith an overall stretch ratio of at least about 1.5:1 as measured fromthe mid-point of on junction-forming zone to the mid-point of theadjacent junction-forming zone in the SD, to form orientated SD strands;in the geogrid so produced, the mid-point of the junction zone beingsubstantially thicker than the mid-point of any oriented strand enteringthe junction zone, and the edge of the length of the crotchinterconnecting adjacent sides of adjacent oriented PD and oriented SDstrands being oriented in the direction running around the crotch withthe orientation ratio decreasing significantly as one passes around thecrotch edge either from the oriented PD strand or from the oriented SDstrand, the crotch edge either a) having an unoriented part, or b) thethickness of the least oriented part of the crotch edge being reduced,or the length of the least oriented part of the crotch edge increased,by not more than about 20% by the action of the stretching.
 21. Asubstantially uniplanar biaxially-stretched integral plastics materialmesh structure geogrid for use in a geotechnical construction, having agreater strength in a primary direction (PD) than in a secondarydirection (SD) substantially at right angles to the PD and having afirst face and a second face, made from a starting material which has athickness of at least about 2 mm, and comprising oriented PD strands andoriented SD strands, interconnected by junctions whose mid-pointthickness is substantially greater than the mid-point of any orientedstrand entering the junction, adjacent sides of adjacent oriented PD andoriented SD strands being interconnected by crotches whose edges for atleast part of their length are oriented with orientation running in thedirection running around the crotch, said PD and SD strands and saidcrotches defining mesh openings substantially free of fibrils, theorientation of the PD strands extending into and through the junctions,and the junctions comprising a central zone which is thicker thanthinner zones in the junction adjacent the ends of oriented PD andoriented SD strands, the junction central zone having on each facethereof a narrow generally diagonally-extending projection at the cornerextending outwards between said thinner zones, continuing through thecrotch between the oriented PD and oriented SD strands and running intothe crotch edge, with no point on a line of maximum thickness on thegeogrid from the junction mid-point to the crotch edge having athickness of less than about 80% of the thickness of the junctionmid-point.
 22. A geotechnical construction, comprising a mass ofparticulate material and a reinforcing means therefor, said reinforcingmeans comprising at least one generally horizontally-extending layer ofa geogrid produced by the method of claim 1, said geogrid having upperand lower faces, said PD strands and said SD strands definingtherebetween mesh openings, said geogrid being embedded in said mass ofparticulate material with portions of said mass of particulate materialbelow said geogrid, portions of said mass of particulate material abovesaid geogrid, and portions of said mass of particulate material withinsaid mesh openings, so that portions of said particulate material are indirect contact with said upper and lower faces of said geogrid and withportions of said geogrid which define said mesh openings, whereby saidmass creates tensile forces in said PD strands and said geogrid has goodslip resistance properties with respect to said particulate material.23. A method of constructing a geotechnical construction, comprising:providing a mass of particulate material and a reinforcing meanstherefor, said reinforcing means comprising at least one generallyhorizontally-extending layer of a geogrid produced by the method ofclaim 1, said geogrid having upper and lower faces and said PD and SDstrands defining therebetween mesh openings; and embedding said geogridin said mass of particulate material with portions of said mass ofparticulate material below said geogrid, portions of said mass ofparticulate material above said geogrid, and portions of said mass ofparticulate material within said mesh openings, so the portions of saidparticulate material are in direct contact with said upper and lowerfaces of said geogrid and with portions of said geogrid defining saidmesh openings; whereby said mass creates tensile forces in said PDstrands and said geogrid has good slip resistance with respective tosaid particulate material.
 24. A method of producing abiaxially-stretched plastics material geogrid for use in a geotechnicalconstruction, having a greater strength in a primary direction (PD) thanin a secondary direction (SD) substantially at right angles to the PD,comprising: providing a substantially uniplanar plastics startingmaterial which has a thickness of at least about 2 mm and has a patternof through-holes on a substantially square or rectangular grid, the SDpunch-out being less than about 60%, the axes of said grid beingsubstantially parallel to the PD and to the SD respectively, the sidesof at least some PD end portions of said holes being defined bycrotch-forming zones in which, as seen in plan view as one progressestowards the PD end of the hole, each said crotch-formning zone has: (i)a first part having a side which is progressively more inclined to thePD and is defined at least in part by a curve which is concave withrespect to the hole; (ii) a second part having a side which isprogressively less inclined to the PD and is defined at least in part bya curve which is convex with respect to the hole; and (iii) a third parthaving a side which is progressively more inclined to the PD and isdefined at least in part by a curve which is concave with respect to thehole; applying PD stretch to form oriented PD strands and to apply someorientation to the junction-forming zones so that orientation extendsinto and through the junction-forming zones; and applying SD stretchwith an overall stretch ratio of at least about 1.5:1 as measured fromthe mid-point of one junction-forming zone to the mid-point of theadjacent junction-forming zone in the SD, to form oriented SD strands;the geogrid so produced having a first face and a second face, and insaid geogrid, the junction zone forming a junction which interconnectstwo oriented PD strands entering the junction from opposite sides andtwo oriented SD strands approaching or entering the junction from twoother opposite sides, the junction comprising a central zone which isthicker than thinner zones in the junction adjacent the ends of theoriented PD and oriented SD strands and the central zone beingsubstantially square or rectangular and having a narrow projection ateach corner thereof extending outwards between said thinner zones,continuing through the crotch between the respected oriented PD andoriented SD strands and running into the crotch edge, the mid-point ofthe junction zone being substantially thicker than the mid-point of anyoriented strand entering the junction zone, and the edge of the lengthof the crotch interconnecting adjacent sides of adjacent oriented PD andoriented SD strands being oriented in the direction running around thecrotch with the orientation ratio decreasing significantly as one passesaround the crotch edge either from the oriented PD strand or from theoriented SD strand, the crotch edge either a) having an unoriented part,or b) the thickness of the least oriented part of the crotch edge beingreduced, or the length of the least oriented part of the crotch edgebeing increased, by no more than about 20% by the action of stretching,and the action of stretching not reducing the thickness of any pointalong generally diagonally-extending lines of maximum thickness on thebiaxially-stretched mesh structure from the mid-point of the junctionzone to the crotch edges to such an extent that the ratio of finishedthickness to starting thickness at that point is less than about 80% ofthe ratio of finished thickness to starting thickness of the junctionzone mid-point.
 25. The method of claim 1, wherein said startingmaterial defines a plane and said holes are defined by sides whichextend at right angles to said plane.